This invention relates to wind turbines.
Over the past several years, there has been much research and development in the field of wind turbine-based electrical power generation. As the size and power generating capabilities of wind turbines has increased, the need for reliable control capabilities has increased accordingly. Specifically, the ability to compensate for varying wind conditions--from the ground to the top of the rotor assembly as well as from one point in time to the next--is highly desirable. The ability to shut down the rotor in the event the power load on the rotor is suddenly lost, to prevent catastrophic overspeed of the rotor, is critical.
Some prior wind turbine configurations have used pitch control schemes wherein the pitch of the entire rotor blade is varied. Other configurations have incorporated spoilers or ailerons at the trailing edges of the rotor blades. The spoilers or ailerons extend for a portion of the span of each rotor blade, typically near the outer end of the blade.
As shown in FIGS. 1A and 1B, a typical wind turbine assembly includes a rotor assembly 10 mounted to a gearbox assembly 12, which is in turn supported at the top of a tower structure 14. Through any of a variety of mechanisms known in the art, the combined rotor/gearbox assembly is rotated about the tower 14 such that the rotor faces into the oncoming free-stream wind. The free-stream wind is represented by the velocity vector V.sub.W, which faces into the page in FIG. 1B. The rotor assembly 10, which may in some instances be constructed with a coning angle .DELTA., typically has two or three rotor blades 20.
As shown in FIG. 2, the free-stream wind strikes the high pressure surface 22 of rotor blade 20 and is deflected towards the trailing edge 24, as indicated by curved arrows 25. The force of the air being deflected causes the rotor assembly 10 to begin rotating at an angular frequency .OMEGA., and the tangential velocity V.sub..OMEGA. at any radial position r along the rotor blade 20 is equal to r.multidot..OMEGA.. The rotor blade "sees" a local relative wind, represented by the velocity vector V.sub.rel which is equal to the sum of the local tangential velocity V.sub..OMEGA. and the free-stream wind velocity V.sub.W, which strikes the rotor blade 20 at a local angle of attack .alpha.. It will be appreciated that, for a given free-stream wind velocity V.sub.w and angular frequency .OMEGA., the tangential velocity V.sub..OMEGA. and, hence, local relative wind and local angle of attack .alpha. will vary along the length of the rotor blade 20.
As the rotor blade 20 moves through the air, with a local relative wind having velocity V.sub.rel at any given position r, lift L is generated normal to the local relative wind. The lift L has a component L.multidot.sin(.theta.) in the direction of rotation, where .theta., the relative wind angle, is equal to the local angle of attack .alpha. plus the local pitch or twist angle .phi., the angle between the chord line 26 and the plane of rotation 16. Drag D, parallel to the local relative wind, has a component D.multidot.cos(.theta.) opposite the direction of rotation. The net aerodynamic force in the direction of rotation, referred to as the suction force S and equal to L.multidot.sin(.theta.)-D.multidot.cos(.theta.), imparts a torque on the rotor assembly 10. The angular velocity .OMEGA. will increase, under the influence of the torque, until the suction force S is balanced by retarding forces, e.g., power load and friction.
As noted above, the use of ailerons at the trailing edge of the rotor blades has been investigated for regulating the performance of wind turbines. The ailerons are located at the outer region of the rotor blade and typically have a length on the order of 30% of the total blade length. Configurations previously tested, as illustrated in FIGS. 3A, 3B, 3C, and 3D, have typically employed ailerons 32 which are little more than discrete, segmented portions of the rotor blade 20 itself. These ailerons, usually comprising about 20% to 38% of the total rotor blade chord, have been hinged to the main section 34 of the rotor blade 20 along the low pressure surface 28. They have often been attached via a hinge 36 located right at the leading edge 38 of the aileron 32, as shown in FIGS. 3A and 3B. Alternatively, in other configurations as shown in FIGS. 3C and 3D, the hinge 36 has been mounted at the end of an extension plate 40 such that a flow gap 41 is formed between the aileron 32 and the main section 32 of the rotor blade 20 as the aileron 32 is rotated. The flow gap 41 allows air to flow from the high pressure surface 22 of the rotor blade to the low pressure surface 28 of the rotor blade.
In both of these configurations, deflection of the aileron 32 changes the lift generated by the rotor blade by modifying the camber of the rotor blade 20 and, especially where the configuration provides a flow gap 41, by disrupting the airflow over the low pressure surface 28. Additionally, deflection of the aileron 32 increases drag on the rotor blade 20. Given a large enough deflection, the aileron 32 can be used to slow substantially the rotation of the rotor assembly 10. It has not previously been possible, however, to stop the rotation entirely using just ailerons because the negative suction force generated by the aileron portion of the blade has been insufficient to overcome the positive suction force generated over the non-aileron sections of the blade.
Furthermore, where the hinge 36 is located along the low pressure surface 28, either at or slightly behind the leading edge 38 of the aileron, deflecting the aileron 32 moves the center of mass of the aileron transverse to the direction of rotation of the rotor blade 20. For a large scale wind turbine, i.e., one having a rotor diameter on the order of sixty feet or more, rotating at a frequency .OMEGA. on the order of fifty revolutions per minute or more, gyroscopically induced moments on the aileron 23 can be quite large. These moments lead to excessive "wear and tear" on the hinge 36, as well as on the actuation mechanism used to deflect the aileron 32.